1. Field of the Invention
The present invention relates generally to the field of seismology, and more particularly to the processing of signals from dual-sensor arrays to eliminate undesired seismic energy.
2. Description of the Related Art
The field of seismology focuses on the use of artificially generated elastic waves to locate mineral deposits such as hydrocarbons, ores, water, and geothermal reservoirs. Seismic exploration provides data that, when used in conjunction with other available geophysical, borehole, and geological data can provide information about the structure and distribution of rock types and their contents.
Most oil companies rely on seismic interpretation for selecting the sites in which to invest in drilling oil wells. Despite the fact that seismic data is used to map geological structures rather than finding petroleum directly, the gathering of seismic data has become a vital part of selecting the site of exploratory and development wells. Experience has shown that the use of seismic data greatly improves the likelihood of a successful venture.
Seismic data acquisition is routinely performed both on land and at sea. At sea, seismic ships may trail a streamer or cable behind the ship as the ship moves forward. Alternatively, the ships may deploy receivers and allow the receivers to rest on the ocean floor. This is known as an on-bottom cable (OBC) operation. In OBC operations a second ship typically deploys a seismic source with which to generate seismic waves. Processing equipment aboard the two ships controls the operation of the seismic source and receivers and processes the acquired data.
The cables normally include electrical or fiber-optic cabling for connecting the receivers to seismic monitoring equipment on a ship. The receivers may be uniformly spaced along the cables, and the cables may typically be several miles in length. Data is digitized near the receivers and transmitted to the ship through the cabling.
Various types of seismic sources are used to generate seismic waves for the seismic measurements. To determine the location and configuration of subsurface structures, the seismic source emits seismic waves that travel through the water and earth. These seismic waves generate reflections at various interfaces, including the interfaces created by the presence of the subsurface structures. By determining the length of time that the seismic waves take to travel from the seismic source to the subsurface structure and back to the seismic receivers, an estimate of the distance to subsurface structure can be obtained. By estimating the distance from the various receivers to a subsurface structure, the geometry or topography of the structure can be determined. Certain topographical features are indicative of oil and/or gas reservoirs.
Two primary types of receivers are used in seismology: hydrophones and geophones. Most commonly used for marine seismology are the hydrophones, sometimes called marine pressure phones. These receivers detect pressure changes, and are usually constructed using a piezoelectric transducer that generates a voltage proportional to the pressure change it experiences. Geophones are commonly used for land seismology and are particle velocity detectors. Geophones are directional, i.e., they provide outputs that are dependent on the orientation of the sensor, whereas hydrophones are omnidirectional.
Recently, seismologists have begun using dual sensor receivers for OBC operations. Dual sensor receivers include both a hydrophone and a geophone. Signal traces from each sensor are processed and combined so as to produce an improved seismic trace. The directionality of a geophone allows a well designed system to reduce or eliminate ghosts and other signal artifacts from the hydrophone signal. Ghosts are undesirable seismic signals which have at some stage travelled upwards towards the sea surface before travelling down to the receiver.
The simultaneous collection of pressure and velocity information has the general potential to allow separation of upgoing and downgoing energy, and thus perform ghost removal for marine data. In the special case of the receivers being positioned on the sea floor, it should also be possible to remove all energy that is trapped in the water layer (simple water bottom multiple energy). However, this requires scaling one of the signal traces by a factor that is dependent on the reflectivity at the water bottom. Inclusion of effects of the source ghost as well as receiver ghost have led to development of so called "Baccus filter methods" which attempt to remove the source ghost as well as the receiver ghost.
Existing processing methods for reducing ghosts and signal artifacts generally make some combination of the following assumptions: that the seismic waves are vertically incident plane waves, that the geophone response is omnidirectional, that the hydrophone and geophone have similar impulse responses, that the sensors are perfectly coupled, and that the sensors have similar noise characteristics. In fact, some field systems are now manufactured to "balance" the response of the two detector types in an attempt to make these assumptions valid and to avoid any requirement for additional scaling. However, since some of these conditions are deterministic and measurable, it would be desirable to provide for a processing method that does not limit its accuracy by making any of these assumptions. Additionally it would be preferable to combine the signals in a way which improves the signal-to-incoherent-noise ratio in the combined result.
It is noted that no currently available methods address the angular dependence of the reflectivity of the ocean floor. Any attempt to provide a simple scalar correction is at best only a first order approximation, since reflectivity may depend on the angle of incidence. It may be argued that the range of incident angles in seismic data is small enough for the approximation to be valid, but this is unlikely to be true for reflections in shallow water layers.
It is notable that most current methods are designed to combine the signals from a hydrophone and a geophone by simple scaling and summation. It is desirable to make the scalars be dependent on the signal (regardless of the method of determination). We describe here a method of combination which is dependent on Signal to Noise, and additionally may be different for each frequency recorded. The result has the benefit of having better signal to noise characteristics than any simple summation method.